The present invention pertains to alignment systems, and in particular to backside alignment systems and methods for substrate alignment in lithography systems.
In semiconductor manufacturing, the processing steps for fabricating a semiconductor device (e.g., an integrated circuit) involve exposing a substrate, such a semiconductor wafer coated with photosensitive material, using a lithographic exposure system. This exposure requires aligning the substrate residing on a substrate stage (hereinafter, xe2x80x9cchuckxe2x80x9d) to a reticle having a pattern of a particular device layer and residing on a reticle stage. To perform alignment, the lithographic system includes an alignment system, such as that disclosed in U.S. Pat. No. 5,621,813 (referred to hereinafter as xe2x80x9cthe ""813 patentxe2x80x9d), which patent is incorporated herein by reference. After alignment, the reticle is exposed to radiation to which the photosensitive coating is sensitive, to transfer the reticle pattern onto the wafer. This alignment and exposure can be performed on a variety of lithography systems such as step and repeat, projection, contact and proximity systems, for example. Typically, the first of such device layers is aligned to some marking on the wafer, for example, to a flat or notch, as is well known. Subsequent layers are then aligned relative to this first layer and/or to each other through the use of alignment marks printed in the kerf region, i.e., the region between exposure fields. Typical alignment marks include chevrons, squares, crosses and grouped lines of various orientation.
In some lithographic applications, such as micromachining of substrates in the production of pressure transducers, the three-dimensional nature the device being produced requires precise alignment of the structure from the frontside through to the backside of the substrate. In certain cases, the substrate is processed on one side, and then flipped over and processed on the opposite side to create the desired three-dimensional structure. In such cases, frontside and backside alignment must be performed to ensure the three-dimensional structure is properly aligned. For example, if there is a contact running through from the top to bottom sides of the substrate, it must be precisely aligned to the other elements in the device so that it can provide the necessary electrical contact.
There are devices in the prior art that are used to view a substrate from the backside. For example, U.S. Pat. No. 5,821,549 discloses acquiring from the backside of a substrate an IR optical microscope image of a feature formed therein, and aligning the image with a coordinate system of a milling system so that material can be milled away from the backside to expose the selected feature. However, this technique provides no way to align successive patterns on the front side using the marks on the backside.
U.S. Pat. No. 5,985,764 discloses a technique for viewing alignment marks from the backside of a substrate by coating the alignment marks on the frontside of the substrate with an IR-reflective coating. This technique is used to overcome obscuring effects on the alignment marks due to the topography of overlying layers from the various process steps. Unfortunately, the technique requires adding steps to the process for forming the device, which is undesirable.
U.S. Pat. No. 5,929,997 discloses a method for aligning a reticle with a semiconductor wafer that includes simultaneous viewing of the alignment marks on a reticle and on the backside of a wafer through the chuck supporting the wafer. However, the method is fairly complex in that the alignment light must pass through the reticle as well as through the wafer.
Accordingly, there is a need for a simple, robust backside alignment system that can quickly measure a variety of alignment sites on a substrate in order to quickly align a substrate so that it can be processed.
The present invention pertains to alignment systems, and in particular to backside alignment systems and methods for substrate alignment in lithography systems.
In particular, the present invention is a system and method for performing alignment of a substrate using alignment marks on the backside of a substrate supported by a movable chuck. The system includes an imaging optical system arranged such that the movable chuck can position one end of the optical system either adjacent the front surface of the substrate or near the front surface but outside the perimeter of the substrate.
In aspect of the invention, secondary optical systems are arranged within the chuck at the chuck perimeter so as to be in optical communication with corresponding alignment marks. The chuck is movable so that the imaging optical system can be placed in optical communication with the second optical system and image the alignment marks onto a detector. The detector converts the alignment mark images into digital electronic images, which are stored in a computer system. The chuck then moves the substrate to exposure locations based on the calculation.
In another aspect of the invention, a target on the chuck is used to align reticle targets (xe2x80x9ckeysxe2x80x9d) with the alignment marks on the substrate so that a reticle can be imaged onto a substrate using a lithography system having a projection lens. This is facilitated through the use of an alignment target placed on the chuck. A separate alignment system is used to acquire an image of the chuck target and determine the chuck position that aligns the chuck target to each of the reticle alignment key images imaged onto the chuck through projection lens. The chuck target is then moved to a position where it is aligned with the imaging optical system. The distance traveled locates the positions of the reticle alignment keys with respect to the backside alignment targets. These offsets are then used to calculate the positions of the chuck that result in the correct placement of the reticle alignment key images (and thus the reticle pattern) projected onto the substrate.